In what some are calling a ‘fluke,’ a blade detached from a wind turbine in the S.D. desert; but could trend toward larger machines be a factor?

A wind turbine failure in the desert east of San Diego, in which a 170-foot blade fell to earth, represents an extremely rare event amid a trend toward larger and more reliable machines, according to the director of the nation’s leading wind energy research and development facility.

The Ocotillo Wind power plant, a turbine array 70 miles east of San Diego, remained offline Thursday, two weeks after discovery of the detached blade. The manufacturer of the plant’s turbines, Siemens, has been searching for the root cause of the incident, and whether it may be related to blade detachment at a wind farm in Iowa in April. The company did not respond to inquiries about its progress.

Industrial-scale wind turbines have grown steadily bigger over the decades — and more reliable, too, explained Ford Felker, the director of the National Wind Technology Center.

“Thirty years ago, having a blade separate from a turbine was not that uncommon, but there has just been tremendous improvements in the development process for wind turbines,” Felker said, “Both from our ability to analyze how the structures would behave using computer modeling, and in terms of the types of testing that’s required now.”

The turbine blades at Ocotillo are constructed with a glass fiber-reinforced epoxy resin, and attached to a rotor suspended about 240 feet from the ground.

Engineers of composite turbine blades strike a careful balance between lightness and strength, said Felker, a Stanford-educated mechanical engineer with prior experience in aircraft and astronautics industries.

Eking out new production and costs efficiencies, meanwhile, is critical if the industry is to ever compete directly with fossil fuels and nuclear power. That has led to larger and larger turbines, with some blades longer than 250 feet.

To ensure structural integrity, turbine makers typically perform a combination of two types of stress tests. An “ultimate load test” mimics the maximum force a blade would ever experience in the outdoors — and then some. Fatigue testing generally replicates the oscillatory vibrations a turbine experiences day-in and day-out over a 20-year period. Sometimes, the ultimate load test is repeated before and after.

Photographs of the broken turbine at Ocotillo suggest the blade separated entirely at the joint with the rotor, and joints present a special challenge in any complex mechanical engineering design.

But here, too, the industry has come up with a reliable fix using a series of T-shaped bolts, said Felker, who oversees about 100 researchers in Louisville, Colo., at the largest wind energy research and development center in the U.S.

“Really this particular area of the blade has not been an area of particular challenge for the industry for some time,” said Felker, who has not been involved in the Siemens analysis. “It sounds a little bit like a fluke — that this is very unusual.”

At the same time, structural reliability standards for wind turbines are much more loosely regulated than the high-stakes aviation industry, where lives are almost always at risk.

That is as it should be, said Felker, describing reliability standards that are reinforced by the marketplace. The wind industry has struck a “pretty good balance” between cost and reliability.

“If you have a system that’s ultrareliable but too expensive, well, it doesn’t do well in the marketplace,” he said. “Conversely, if you have a system that is really cheap, but falls apart, well, you might do well in the marketplace for a year or two, but ultimately your business will fail.”

After the blade break, Siemens, a leading worldwide manufacturer of turbines, curtailed the operation of 700 similar turbines in use worldwide, allowing some to remain in operation at slower speeds. It was unclear if the restrictions still remain in place.